The invention relates to an electrically resistive structure comprising a substrate which is provided on at least one side with a first and a second film of resistive material, the materials of the first and second films being mutually different.
An electrically resistive structure of this type is known from European Patent Specification EP-B 0 175 654, wherein an Al.sub.2 O.sub.3 substrate is consecutively provided with resistive films of cermet and NiCr. Since the sheet resistance of the NiCr film is significantly lower than that of the cermet film, such a structure may be viewed as an in-plane parallel arrangement of a high-ohmic resistor (cermet) and a low-ohmic shunt (NiCr).
When a structure of this type is embodied as a connecting double strip between two terminal points on the substrate, its in-plane electrical resistance between those points can be successively increased by, for example:
Increasing the path-length of the structure, or decreasing its width; PA1 Etching away the low-ohmic shunt strip; PA1 Increasing the path-length of the remaining high-ohmic resistor strip, or decreasing its width.
Such procedures can be controllably performed with the aid of well-known selective masking and etching techniques, such as elucidated for example in the book "The Chemistry of the Semiconductor Industry", edited by S. J. Moss and A. Ledwith, ISBN 0-216-92005-1, Blackie & Son, London (1987), in particular in chapters 9 and 11. In this manner, it is possible to produce a substrate having on its surface well-defined strips of resistive material which demonstrate a variety of accurately trimmed resistances. When provided with external electrical contacts, such strips serve as integrated resistors, so that it is possible to achieve an entire integrated thin film resistor network upon the substrate.
In the interest of maximising the range of possible resistance values which can be achieved on any given specimen substrate, it is advantageous if the sheet resistances of the materials of the first and second films differ by at least one order of magnitude (i.e. factor of ten), and preferably by several orders of magnitude (such as a factor of 1000). In addition, the achievement of well-defined resistance tolerances over a relatively wide temperature range requires the resistive structure to have a stabilised Temperature Coefficient of Resistance (TCR).
For purposes of clarity, the sheet resistance R.sub..quadrature. of a film of thickness t comprising a material of electrical resistivity .rho. is here defined as R.sub..quadrature. =.rho./t.
The inventors have observed that the TCR of various resistive materials in a single-layer configuration can generally be significantly stabilised by subjecting those materials to an annealing step, typically performed at a temperature of about 350-550.degree. C. in a gaseous atmosphere (comprising, for example, air, nitrogen or argon). In the case of a bilayer resistive structure, however, it has unfortunately been observed that subjection of the structure to such annealing treatment generally causes deterioration of the properties of at least one of the structure's component resistive materials. In particular, the TCR-value of at least one of the materials may change significantly from that which was originally intended. In addition, annealing can lead to a substantial reduction of the difference in sheet resistance between the first and second resistive films, particularly when this difference is originally large (e.g. factor 100-1000).